1. Field of the invention
This invention relates to a new copper-oxide high T.sub.c (above 100 K) superconductor, Tl-Sr-Ca-Cu-oxide.
2. Description of the Background Art
Since 1987, there has been considerable research on high T.sub.c copper oxide superconductors. Superconductors with T.sub.c 's greater than 77 K include, for example, Y(RE)Ba.sub.2 Cu.sub.3 O.sub.7, Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.x and Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, this latter compound having the highest known T.sub.c (C. C. Torardi, M. A. Subramanian, J. C. Calabrese, J. Gopalakrishnan, K. J. Morrissey, T. R. Askew R. B. Flippen, U. Chowdhry, and A. W. Sleight. Science 240, 631-634 (1988)). While these compounds are complex layered structures, only a very limited number of combinations of elements are favorable for superconductivity. In addition to Cu and O, which all known high T.sub.c compounds contain, the known favorable combinations are 1) a rare earth element (RE) and Ba, 2) Bi, Sr and Ca and 3) Tl, Ba and Ca. Numerous attempts to expand the list of combinations have failed. Although Ba and Sr are chemically similar, attempts to substitute Sr for Ba and vice-versa in the above combinations have been unsuccessful. For example, attempts to substitute Sr for Ba in Tl-Ba-Ca-Cu-O systems to fabricate superconducting Tl-Sr-Ca-Cu-O systems using current conventional ceramic processing techniques have all failed. Although Sr has been partially substituted for Ba in Tl compounds, it has resulted in a substantial lowering of T.sub.c (M. A. Subramanian, C. C. Torardi, J. Gopalakrishnan, P. L. Gai, J. C. Calabrese, T. R. Askew, R. B. Flippen, and A. W. Sleight. "Bulk Superconductivity Up To 122 K In The Tl-Pb-Sr-Ca-Cu-O System"). One probable reason for this is that Ba and Sr have different ionic sizes which causes the analog compounds to be energetically unfavorable.
In addition to the inability to substitute Sr for Ba in forming superconducting compounds, there is also a problem in forming superconducting Tl containing compounds. A major problem with any Tl-based compound is its toxicity. Tl has a relatively high vapor pressure at temperatures needed to react superconducting compounds.